KVM: PPC: Book3S HV: Fix duplication of host SLB entries

Since commit 6964e6a ("KVM: PPC: Book3S HV: Do SLB load/unload
with guest LPCR value loaded", 2018-01-11), we have been seeing
occasional machine check interrupts on POWER8 systems when running
KVM guests, due to SLB multihit errors.

This turns out to be due to the guest exit code reloading the host
SLB entries from the SLB shadow buffer when the SLB was not previously
cleared in the guest entry path.  This can happen because the path
which skips from the guest entry code to the guest exit code without
entering the guest now does the skip before the SLB is cleared and
loaded with guest values, but the host values are loaded after the
point in the guest exit path that we skip to.

To fix this, we move the code that reloads the host SLB values up
so that it occurs just before the point in the guest exit code (the
label guest_bypass:) where we skip to from the guest entry path.

Reported-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Fixes: 6964e6a ("KVM: PPC: Book3S HV: Do SLB load/unload with guest LPCR value loaded")
Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -1557,6 +1557,24 @@ mc_cont:
 	ptesync
 3:	stw	r5,VCPU_SLB_MAX(r9)
 
+	/* load host SLB entries */
+BEGIN_MMU_FTR_SECTION
+	b	0f
+END_MMU_FTR_SECTION_IFSET(MMU_FTR_TYPE_RADIX)
+	ld	r8,PACA_SLBSHADOWPTR(r13)
+
+	.rept	SLB_NUM_BOLTED
+	li	r3, SLBSHADOW_SAVEAREA
+	LDX_BE	r5, r8, r3
+	addi	r3, r3, 8
+	LDX_BE	r6, r8, r3
+	andis.	r7,r5,SLB_ESID_V@h
+	beq	1f
+	slbmte	r6,r5
+1:	addi	r8,r8,16
+	.endr
+0:
+
 guest_bypass:
 	stw	r12, STACK_SLOT_TRAP(r1)
 	mr 	r3, r12
@@ -2018,23 +2036,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
 	mtspr	SPRN_LPCR,r8
 	isync
 48:
-	/* load host SLB entries */
-BEGIN_MMU_FTR_SECTION
-	b	0f
-END_MMU_FTR_SECTION_IFSET(MMU_FTR_TYPE_RADIX)
-	ld	r8,PACA_SLBSHADOWPTR(r13)
-
-	.rept	SLB_NUM_BOLTED
-	li	r3, SLBSHADOW_SAVEAREA
-	LDX_BE	r5, r8, r3
-	addi	r3, r3, 8
-	LDX_BE	r6, r8, r3
-	andis.	r7,r5,SLB_ESID_V@h
-	beq	1f
-	slbmte	r6,r5
-1:	addi	r8,r8,16
-	.endr
-0:
 #ifdef CONFIG_KVM_BOOK3S_HV_EXIT_TIMING
 	/* Finish timing, if we have a vcpu */
 	ld	r4, HSTATE_KVM_VCPU(r13)